The researchers evaluated 28 antivirus products against three criteria: protection, repair and usability. Products could earn up to six points in each category for a possible total of 18 points. After tallying the results, AV-Test reported that "Bitdefender, Kaspersky Lab and Symantec lead the field while the protection packages from Avast, F-Secure and GData share fourth place."

Still, all of the products earned top scores of 15.2 points or more -- which stood in sharp contrast to the performance of Microsoft's free offerings. "The test results of these [six] products alone are all nearly five points higher than the overall result obtained by Microsoft's Windows Defender or Security Essentials when used together with the Windows Firewall," reported AV-Test. "This proves that the use of external security solutions can lead to a massive improvement when it comes to system protection."

Interestingly, the top-ranked applications weren't always the best at stopping malware, as measured by the lab's "protection" tests. "The suites from Bitdefender, F-Secure and Kaspersky all did the best job in this category, achieving detection rates of 100%, while the best free programs, namely those from Avast and AVG, were only able to make it to eighth and twelfth place respectively," said AV-Test. "The Windows Defender provided by Microsoft in its operating system set a very low benchmark value with a detection rate of just 79%."

The protection tests were designed to test each product's real-world detection capabilities, and involved subjecting each product to 400 pieces of brand-new -- aka zero-day -- malware.

The products also were tested using a "reference set" comprising 60,000 pieces of malware. "The malware in the [reference] set is already up to four weeks old," said AV-Test. "Good programs are therefore always able to identify 100% of the malware on this list." Furthermore, AV-Test said products only failed the malware detection test if both their scanner and any additional on-demand detection capabilities couldn't identify the malware. "After all, most of the protection packages not only feature basic detection functions, but also a number of other important tools that they use to identify malware," said the research firm.

Microsoft Windows Defender, however, only detected 97% of the reference set, putting it in last place compared to 23 other products that were also tested three different times in six months. In fact, the only other tested products that failed to achieve a 100% reference-set detection rate were Check Point's ZoneAlarm Free Antivirus and Firewall, and AhnLab's V3 Internet Security, as well as K7's Total Security, although that product was only tested twice between January and June.

Microsoft's free endpoint security software, however, did earn top marks in usability, which only five other tested products managed to equal.

AV-Test also examined the impact of the endpoint security software on system load, and found that malware-stopping power comes at a price: system performance. "Although the best programs in the 'Protection' category also achieved excellent results in this 'System Load' category, none of them were able to score the maximum total of six points," reported AV-Test. "This test category is proof that high security comes at the expense of a certain amount of system performance." On average, the top 10 products earned an average of 4.0 points (out of 6.0) for system load, while the top-ranked product, from Bitdefender, earned 5.2.

Interestingly, AV-Test found variations in the tested programs' effectiveness depending on the version of Windows being used in the test. Overall, the research firm found that on Windows 8, tested antivirus products correctly detected zero-day malware 95% of the time, on average, followed by 93% for Windows XP and 92% for Windows 7.